Cupronickel is an alloy of copper, nickel and stengthening impurities. It does not corrode in seawater, because its electronegativity is adjusted to be neutral with regard to seawater.
Cupronickel is used for marine hardware, and sometimes for the propellers, crankshafts and hulls of premium tugboats, fishing boats and other working boats.
The most ubiquitous use, from the point of view of the average person, is that most of the silver-coloured modern circulation coins are Cupronickel. A typical mix is 75% copper, 25% nickel, and a trace amount of manganese.
It is used in thermocouples, and a 55% copper/45% nickel alloy is used to make very accurate resistors.